Pathogen reduction system used in treating wastewater

ABSTRACT

A pathogen reduction system for a sludge treatment process that employees multiple reactors operating in a batch mode to reduce pathogen levels in the sludge while providing a continuous flow of pathogen reduced sludge to a digester. In addition, the present invention entails an efficient heat exchanger for heating and cooling the sludge being treated in the above process. In particular, the heat exchanger is of the counterflow type and includes a helical coil confined in an annular chamber for transmitting one media and wherein the helical coil is spaced so as to form another helical flow channel that is defined by the annular chamber and the helical coil itself. Consequentially, one media is directed through the helical coil while another media is directed through the flow channel that is defined in part by the helical coil.

FIELD OF THE INVENTION

The present invention relates to wastewater treatment systems and, moreparticularly, to a pathogen reduction system for treating sludge.

BACKGROUND OF THE INVENTION

Wastewater treatment systems are widely used throughout the world toremove nutrients such as nitrogen and phosphorous from the wastewater aswell as destroy pathogens and viruses within the resulting sludge beforethe purified effluent is discharged and before the final treated sludgeis removed from the wastewater treatment system.

Typically, in a wastewater treatment system, wastewater influent isdirected through a series of a secondary treatment zones and subjectedto various forms of treatment such as, for example, anaerobic, aerobic,and/or anoxic treatment. After such treatment, the wastewater isdirected to a final clarifier which separates sludge in the wastewaterfrom purified effluent. The purified effluent is discharged into astream or lake, for example, while the sludge from the final clarifieris returned to the head of the activated sludge system and mixed withthe influent wastewater to form what is commonly referred to as mixedliquor. Throughout the wastewater treatment process, the sludge from thefinal clarifier is recycled through the activated sludge system. Thebiomass or microorganisms associated with the recycled sludge act toeffectively remove nutrients such as nitrogen and phosphorous and reduceBOD and other contaminant levels within the wastewater being treated.

However, the sludge being recycled through the activated sludge systemhas to be continually removed or wasted from the process. In addition,depending on a number of factors such as the contaminant levels in thewastewater influent, certain amounts of primary sludge may be removedduring primary treatment without being processed through the activatedsludge system. This primary sludge is then mixed with the secondarysludge wasted from the activated sludge process and the mixture issubjected to further treatment where contaminants are removed orseparated from the sludge. In a typical wastewater treatment system,this sludge mixture is directed to a digester where the sludge istreated and cleaned by removing pathogens and volatile solids. Usually,one of the most convenient methods of disposal of the resulting sludgeis, for example, by land applications.

Existing wastewater facilities are often designed and built to handle aparticular processing capacity and to produce a certain quality ofeffluent. As technology improves or as conditions change over time, suchas where the quality of the wastewater deteriorates or where theprocessing capacity of the system is exceeded, these wastewaterfacilities become relatively inefficient.

In addition, many treatment processes for wastewater are batch-typeprocesses. This means that large tanks are necessary in order to processthe large batches of wastewater where these tanks are sized relative tothe capacity of the treatment plant. Accordingly, the throughput of aconventional wastewater treatment system is limited by the capacity ofthe tanks and the time required for each batch process in the wastewatertreatment procedure.

Batch processing of sludge also create a number of additional problems.For instance, treated sludge resulting from a batch process performed ina large tank may exhibit nonuniformities which may be attributed to suchcauses as poor mixing of treatment chemicals with the sludge or, where aheat treatment is used, to improper distribution of heat within thesludge. Excessive agitation of the sludge or extended sludge retentiontime may often be necessary to compensate for the shortcomings in batchprocesses, but these measures are not always effective. Furthermore,pipes, pumps, valves, and associated equipment used for directing thesludge through the treatment system spend a significant amount of timein a dry state after batches of sludge have been transferred to theprocessing tanks. Thus, any residual sludge remaining in theseperipheral items will dry and leave a residue during each subsequent drystate, leading to frequent, difficult, and costly maintenance of theseparts.

On the other hand, experience has shown that some of the processes in awastewater treatment system are actually more efficient when operated ina continuous manner instead of being subject to batch processing. Forexample, anaerobic digesters are generally more efficient when exposedto a continuous flow of sludge. This way, the microorganisms which areoperative during the anaerobic treatment are maintained in a continuousactive state and are thus more effective in removing contaminants fromthe sludge. In contrast, in a batch process where the activity of themicroorganisms tapers off near the end of the digestion process,additional time is required in a subsequent batch of sludge torejuvenate the microorganisms to an effective level. That is, batchprocessing limitations again result in additional process time fortreating the sludge.

The incompatibility between batch and continuous flow processes isevident particularly in a wastewater treatment system which must beexpanded to meet increased processing demands. Since these plants aretypically arranged to operate in a batch mode, large additional batchtanks are necessary and the physical land space required for this newequipment is often measured in terms of acres. In addition, the costsinvolved in such an undertaking are often relatively high when comparedto the limited flexibility for future expansion gained for theinvestment. Furthermore, the commensurate increase in operating andmaintenance costs combined with the other shortcomings suggests that amore flexible and efficient alternative is needed.

SUMMARY OF THE INVENTION

The present invention relates to a pathogen reduction system for asludge treatment process that uses multiple reactors operating in batchmode to reduce pathogen levels in the sludge while providing acontinuous flow of pathogen-reduced sludge to a digester. This processsignificantly reduces the time required to treat the sludge in thedigester, in some cases up to a 50% reduction in time, by removingpathogens from the sludge prior to the digestion process. In addition,the present invention requires relatively little equipment to accomplishits purpose, resulting in a significantly lower cost, a smallerfootprint or space requirement, and greater flexibility for futureexpansion as compared to other wastewater treatment processes.

In order to achieve a continuous flow of sludge to the digester, thepresent invention uses multiple reactors operating in a batch mode andconnected in parallel to a common input and a common output line for thesludge being treated. Each reactor is operable to assume a feeding mode,where the reactor is being filled with sludge; a holding mode, where thereactor is holding and treating the sludge; and a discharge mode, wherethe reactor is discharging the sludge to the digester. In a preferredembodiment, at least three reactors are used wherein, at any one time,at least one reactor is in the feeding mode, another reactor is in thestorage mode, and a third reactor is in the discharge mode. Since eachreactor proceeds from the feed mode to the holding mode and then to thedischarge mode in a continuous cycle, each successive reactor must bestaggered by one mode with respect to the preceding reactor. That is, ifthe time periods for the feeding, holding, and discharge modes betweensuccessive reactors are all the same and these modes are staggeredbetween reactors, at least one reactor will always be in the dischargemode, thereby providing a continuous flow of sludge to the digester.

In order to achieve the desired result of synchronization between modechanges in successive reactors, the selected time period for each modemust correspond to the holding time necessary in a single reactor toreduce the pathogen levels in the sludge to meet a selected standard. Atthe same time, temperature is an additional factor which must beconsidered and the sludge is accordingly heated to a minimum temperaturebefore being transferred to the reactors. Thus, for example, experiencehas shown that sludge heated to approximately 65 to 75° C. requiresapproximately 1 hour of retention time in order to reduce the levels ofpathogens to meet certain standards set by the EPA. Accordingly, insystems where three reactors are used, the reactors may be set for 1hour in the feed mode, 1 hour in the holding mode, and 1 hour in thedischarge mode and the transitions between modes are synchronizedbetween successive reactors.

A new efficient heat exchanger particularly for heating and cooling thesludge in a sludge treatment process has also significantly contributedto the effectiveness of the present invention. The heat exchanger issimilar in general operation to a dual coil heat exchanger that includesconcentric helical coils to separate two counter-flowing liquid mediums.However, in the present invention, one media flow path is formed by ahelical flow channel that is defined by an inner and outer sleeveencasing a single tubular helical coil. That is, the inner and outersleeves and the spaces between successive coils of the tubular helicalcoil define the helical flow channel. In essence, this heat exchangerdesign retains the efficiency of a conventional dual coil heatexchanger, but is structured to handle the flow of viscous sludgethrough the coils while being relatively simple to clean and maintain.

The use of parallel processing reactors operating in batch mode so as toprovide a continuous outflow of sludge to the digester, combined withthe use of the new heat exchanger design to heat and cool the processedsludge, results in a very compact and efficient pathogen reductionsystem.

In some cases, the pathogen reduction system of the present inventioncuts the digestion time required to treat the sludge in half, resultingin a greatly increased throughput capacity for the sludge treatmentsystem with minimal equipment and space requirements. Thus, increasedprocessing capacity with minimal equipment addition is another advantageof the present invention.

From the compact size and the minimal mechanical equipment requirementsof the pathogen reduction system, other advantages naturally follow. Forexample, a lower initial cost, lower operating and maintenance costs,and flexibility to expand a sludge treatment system in the future aresome of the further advantages of the present invention.

It is therefore an object of the present invention to provide anefficient and cost effective pre-pasteurization process for treatingsludge.

Another object of the present invention is to provide apre-pasteurization process that effectively reduces digester retentiontime thereby increasing the capacity of the digester.

Another object of the present invention is to provide a combinedpre-pasteurization and digestion process that provides for an efficientand high degree of heat recovery.

Still a further object of the present invention is to provide a combinedpre-pasteurization and digestion process that uniformly loads thedigester and provides a constant production of biogas.

Another object of the present invention is to provide a heat exchangerdesign for handling sludge-to-sludge media which can be quickly andeasily cleaned and maintained.

Other objects and advantages of the present invention will becomeapparent and obvious from a study of the following description and theaccompanying drawings which are merely illustrative of such invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the pathogen reduction system ofthe present invention.

FIG. 2 is a schematic illustration of the pathogen reduction system ofthe present invention showing a particular process for reducing pathogenlevels in sludge prior to the sludge being directed to a digester.

FIG. 3 is a perspective view of the heat exchanger of the presentinvention showing the heat exchanger in an open configuration.

FIG. 4 is a longitudinal sectional view of the heat exchanger shown inFIG. 3.

FIG. 5 is an end view of the heat exchanger shown in FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, there is shown therein a portion of awastewater treatment system that includes a pre-digestion orpasteurization pathogen reduction system that is indicated by thenumeral 35. As illustrated in FIG. 2, the pathogen reduction system 35is disposed upstream of a digester 105 and downstream from a sludgethickening station 25 and a sludge storage tank 20. Disposing thepathogen reduction system 35 upstream with respect to the digester isgenerally preferred. However, it should be pointed out, that thepathogen reduction system 35 could be employed downstream from thedigester 105.

Pathogen reduction system 35 is designed to treat sludge and to removepathogens, such as bacteria, viruses, etc., from the sludge. Generally,the sludge being treated by the pathogen reduction system 35 constituteseither primary or secondary sludge or both. Those skilled in the artwill appreciate that primary sludge is typically separated from influentwastewater during the course of primary treatment. Secondary sludge, onthe other hand, is separated from the wastewater during the course ofsecondary treatment.

In conventional sludge treatment, both the primary and secondary sludgeis combined and after being combined, is directed to a digester such asanaerobic or aerobic digester. Once in the digester, the sludge is heldfor an extended time, sometimes on the order of approximately 30 days,and at a selected temperature level which is typically about 30-35° C.During the process, the digester performs two important functions.First, the digester rids the sludge of pathogens such as bacteria,viruses, etc. Secondly, the digester removes or at least substantiallyreduces, volatile solids found in the sludge.

In the sludge treatment process of the present invention, pathogenreduction is separated from the treatment for volatile solids. In fact,pathogen reduction occurs outside of the digester. In the preferredprocess illustrated and discussed herein, the pathogen reduction system35 removes the pathogens from the sludge prior to the sludge beingadmitted to the digester 105. But after the pathogens have been removedfrom the sludge, the sludge is then directed to the digester 105 whichtreats the sludge over a period of approximately 15 days so as to removevolatile solids from the sludge. Thus, in the end, with the process ofthe present invention, the time period of achieving pathogen reductionand the treatment for volatile solids is reduced to approximately 15days as compared to conventional processes where the same treatment iscarried out entirely within a digester in approximately 30 days.

In the case of the present invention, and referring back to FIG. 2, thesludge is first directed to a thickening station 25 that comprises, forexample, a mechanical drum thickener which removes some water from thesludge and reduces the incoming sludge from approximately 1 to 2% oftotal suspended solids (TSS) by weight to approximately 5% totalsuspended solids. Once the sludge has been thickened, it is transferredfrom the thickening station 25 to a storage or holding tank 30 that isdesigned to continuously meter the thickened sludge to the pathogenreduction system 35.

Briefly reviewing the components and structure of the pathogen reductionsystem 35, it is seen that the same includes a series of reactors thatas a group are referred to by the numeral 45. The series of reactorsincludes three reactors 50, 55 and 60 disposed in parallel relationship.Each reactor is insulated and includes a top driven mixer. In order tocontrol the flow of sludge to and from the reactors there is provided aseries of motor driven flow control valves 62 that are associated withthe respective reactors and which control the flow of sludge to and fromthe reactors. In particular, it is seen that there is provided a controlvalve 62 on both the inlet and outlet side of each of the reactors.

To control the valves 62 and the flow of sludge to and from the reactornetwork 45, a programmable controller 70 would typically be operativelyconnected to the respective control valves 62 so as to operate them in apre-selected and timed sequence. Details of the programmable controller70 are not dealt with herein in detail because such is not per sematerial to the present invention and because programmable controllersare used extensively in wastewater treatment systems to control the flowof wastewater and sludge through various system components. In addition,those people ordinarily skilled in the art understand and appreciate howprogrammable controllers are used to direct and control flow ofwastewater and sludge through various components of wastewater treatmentsystems.

To heat and cool the sludge being directed to and from the reactors,there is provided a heat exchanger network. In the embodimentillustrated herein, the heat exchanger network includes three heatexchangers 80, 85 and 90. As will be appreciated from subsequentportions of this disclosure, the heat exchangers function to heat thesludge being directed to the reactor network 45 and to cool the treatedsludge being directed from the reactors to the digester 105.

As will be discussed further below, the pathogen reduction system 35basically entails heating the sludge to a pre-determined temperature(approximately 55-70° C.) and then storing or holding the sludge for apre-determined amount of time (approximately 1-10 hours) in order toreduce the level of pathogens within the sludge. Once the sludge hasbeen heated and treated by the pathogen reduction system 35, then thetreated sludge is directed to the digester 105 for further treatment, ata lower temperature. This further treatment of the sludge is basicallyaimed at reducing the level of volatile solids found within the sludge.

Now, turning to the process or method of the present invention, sludgefrom the storage reactor 30 is directed into an inlet line 40 of thesludge reduction system 35. Typically, this incoming sludge has atemperature of approximately 10-20° C. Inlet line 40 is communicativelycoupled to two heat exchangers 80 and 85. More particularly, the sludgepassing through inlet line 40 is first passed through heat exchanger 80and after exiting heat exchanger 80, the sludge is directed through adownstream heat exchanger 85. The first heat exchanger 80 functions topreheat the sludge passing in inlet line 40. More particularly, thetreated sludge exiting the network of reactors 45 is directed throughoutlet line 65 into the same heat exchanger 80 and the heat associatedwith the sludge passing through line 65 is used to heat or pre-heat theincoming sludge.

To further heat the incoming sludge in inlet line 40, the downstreamheat exchanger 85 is communicatively coupled to a boiler 90. Boiler 90is powered by the methane or biogas produced by the digester 105. Moreparticularly, the source of the heat that is ultimately transferred tothe sludge is typically water. Water is circulated through the boiler 90and is heated thereby and then directed through the heat exchanger 85 incounter-flow relationship to the sludge passing through the same heatexchanger. In the process discussed herein, the aim is to heat thesludge passing through exchanger 85 to such an extent that the sludgereaching the reactor network 45 assumes a temperature within the rangeof 55-70° C. and more particularly a temperature of approximately 65-70°C.

Once the sludge has been heated by the second heat exchanger 85, it isdirected to the series of reactors 45. Each of the reactors 50, 55 and60 in the embodiment illustrated herein is designed to assume threedifferent modes of operation. The first mode of operation is referred toas a filling mode. This simply means that each reactor is designed toassume a state where sludge is simply being directed into the reactor.In this mode, there is no discharge from the reactor. The second mode ofoperation is referred to as a holding or treating mode. In this mode,the reactor is designed to simply hold the preheated sludge for aselected time period. There is no feed into the reactor and there is nodischarge from the reactor. Finally, the third mode is referred to as adischarge mode. In this mode, sludge is discharged from the reactor andduring the discharge mode, there is no sludge being directed into thereactor.

As pointed out above, each of the reactors 50, 55 and 60, are designedsuch that they continuously cycle through each of the three modes and atany one time during the pathogen reduction process no two reactors willassume the same mode. In other words, during one phase of operation, onereactor will be filling while a second reactor is holding and treatingand while a third reactor is discharging. This occurs for a selected orcertain time period. At the conclusion of that time period the mode ofeach reactor will be changed.

In one process disclosed herein, it is contemplated that each cycle orperiod will last for approximately one hour. Thus, in the way of anexample, during the first hour of operation, reactor 50 would assume afilling mode, reactor 55 would assume a holding mode and reactor 60would assume a discharge mode. During the second hour interval, reactor60 would assume a filling mode, reactor 50 a holding mode, and reactor55 a discharge mode. In the third interval or phase of the process,reactor 55 would assume a filling mode, reactor 60 would assume aholding mode and reactor 50 would assume a discharge mode. Thus, eachreactor sequentially moves from a filling mode to a holding mode andthen to a discharge mode.

In the above example, the incoming sludge is heated to approximately60-70° C. and the heated sludge is held within the reactors forapproximately one hour. It should be noted, however, that the retentiontime and the temperature of the incoming sludge are interdependent.Generally, as the temperature of the incoming sludge is decreased, theretention time in the reactors should be increased to yield a selectedlevel of pathogen reduction. In any event, those skilled in the art willappreciate that both the retention time of the sludge within thereactors and the incoming temperature of the heated sludge can vary.

After the sludge has been held in one of the reactors 50, 55 or 60 for aselected time period and at a selected temperature range, the sludge isdischarged into an outlet line 65. Outlet line 65 feeds into the firstheat exchanger 80 and the heat associated therewith is transferred tothe incoming sludge found in inlet line 40. From the first heatexchanger 80, the outlet line 65 is connected to a third heat exchanger95 for the purpose of cooling the sludge to a temperature ofapproximately 30-35° C. Heat exchanger 95 is operatively connected to acooling source. In the case of this embodiment, treated effluent that isproduced by the wastewater treatment system is directed through heatexchanger 95 and acts to cool the treated sludge prior to its entry intothe digester 105.

Preferably, in the embodiment discussed herein, the treated sludgepassing into the digester 105 is cooled to a temperature ofapproximately 30-35° C. Once in the digester 105, the treated sludge ismaintained therein for approximately 15 days. It has been found that bymaintaining the pre-treated sludge in the digester for a period ofapproximately 15 days at a temperature of approximately 30-35° C.results in the substantial reduction of volatile solids and produces apurified sludge that can be used on agricultural fields, roadsides, etc.This is sometimes referred to as class A sludge.

As discussed above, the individual reactors 50, 55 and 60 generallyperform batch treatment but yet because of the series of reactors andtheir parallel relationship, the output of the reactors as a group iscontinuous. Thus, the digester 105 is fed in a continuous manner.

Heat Exchanger

As discussed above, the pathogen reduction system 35 utilizes a seriesof heat exchangers to heat and cool sludge passing through the system.With particular reference to the drawings, FIGS. 3-5, the heat exchangerdesign utilized in the pathogen reduction system 35 is shown in detail.As illustrated, the heat exchanger is referred to generally by thenumeral 115.

Structurally, the heat exchanger 115 includes a generally cylindricalinner sleeve 120. Formed about opposite end portions of the inner sleeve120 is a flange 120a. Disposed exteriorly of the inner sleeve 120 is anouter sleeve 125. Outer sleeve 125 includes two half sections 125a and125b. Formed about the outer edges of each of the sections 125a and 125bis a partial annular sealing edge 125c (FIG. 3). The function of theannular edges 125c is to engage or mate with the outer flanges 120a ofthe inner sleeve 120 so as to form a generally sealed relationship aboutthe opposite end portions of the heat exchanger 115.

Half sections 125a and 125b of the outer sleeve 125 are supported on anelongated hinge frame 170. A pair of hinge arms 175 are secured aboutopposite ends of the hinge frame 170 and interconnect the hinge frame170 with the respective half sections 125a and 125b. Specifically, thehinge arms 175 permit each of the half sections 125a and 125b of theouter sleeve 125 to move between a closed position (FIG. 5) and an openposition (FIG. 3).

To facilitate the fastening of the half sections 125a and 125b together,each half section includes an elongated flange 125d extending across theedge of the half section opposite the edge that is pivotally connectedto the hinge frame 170. Each flange 125d is turned outwardly andincludes a series of transversely spaced bolt openings. Thus, in theclosed position as shown in FIG. 5, the mating flanges 125d of each halfsection 125a and 125b fit flush such that bolts can be extended throughthe aligned bolts openings in each so as to permit the edges of the halfsections to be tightly secured together.

Secured around the inner sleeve 120 is a helical tubular coil 130. Thehelical tubular coil includes a series of continuous helical segmentsthat are spaced apart so as to define a helical flow channel 150therebetween (FIG. 4). Communicatively connected to opposite ends of thehelical coil 130 is an inlet 135 and an outlet 140. As illustrated inFIG. 4, both the inlet 135 and the outlet 140 extend inwardly of theinner sleeve 120. Therefore, the inlet 135 and outlet 140 effectivelyextend through the inner sleeve 120 and communicate with the helicaltubular coil 130.

The helical flow channel 150 defined by the continuous helical segmentsof the tubular coil 130 also include an inlet 155 and an outlet 160.Both inlet 155 and outlet 160 extend inwardly of the inner sleeve 120.The inlet 155 effectively communicates with the flow channel 150 aboutone end portion of the heat exchanger 115 while the outlet 160communicates with the flow channel 150 about an opposite end portion ofthe heat exchanger 115.

When the half sections 125a and 125b are closed, such as shown in FIG.5, it is appreciated that the flow channel 150 is defined by the helicaltubular coil 130 and the inner and outer sleeves 120 and 125,respectively. In fact, the inner sleeve 130 and the outer sleeve 125 aredesigned such that in the closed position they form a sealedrelationship with the adjacent surfaces of the tubular coil 130. Toachieve this sealed relationship, in one embodiment, the tubular coil130 is expanded, for example, by heating the same, and thereafter theheated coil is placed over the inner sleeve 120. More particularly, thediameter of the helical tubular coil 130 is selected such that it isslightly less than the diameter of the inner sleeve 120. Thus, when thetubular coil 130 is heated it will expand to such a degree that theinner sleeve 120 can be inserted into the tubular coil. Thereafter, thetubular coil 130 cools and closes tightly on the inner sleeve 120.During an ensuing cooling process, the tubular coil 130 contracts so asto form a tight fluid seal between the tubular coil 130 and the innersleeve 120. Other approaches can be utilized to form a fluid tight sealbetween the tubular coil 130 and the inner sleeve 120.

To seal the outer sleeve against the outer surface of the tubular coil130, a sealing material, such as a packing, is interposed between theinner surface of the outer sleeve 125 and the outer surface of thetubular coil 130.

Finally, the heat exchanger 115 is provided with a series of mountingbrackets 165 that permit the entire heat exchanger 115 to be suspendedor supported while in use.

Therefore, in use, the helical flow channel 150 and the tubular coil 130are used to hold and direct counter-flowing fluids within the heatexchanger. From the drawings and the prior discussion, it is appreciatedthat the separation between the continuous helical coil segmentsactually forms a part of the helical flow channel. That is, in use, onemedia directed through the heat exchanger 115 is actually directed inthe defined flow channel 150 while the other media is directed,preferably in a counter-flow relationship, through the tubular coil 130.Thus, in the case of the heat exchanger 115 being utilized to exchangeheat between two sludge streams, as with the case of heat exchanger 80that forms a part of the pathogen reduction system 35 and illustrated inFIG. 2, it follows that one sludge stream will be moving in a helicalfashion through the defined helical flow channel 150 while the othersludge stream will be flowing in a counter direction through the tubularcoil 130.

The heat exchanger 115 is particularly desirable for use in conjunctionwith heavy or viscous fluids that carry suspended solids and which areprone to cause fouling and clogging within the heat exchanger itself.This is because the heat exchanger 115 is very easy to clean andmaintain. Periodically, heat exchangers that are used in wastewater andsludge environments do require cleaning. In the case of the heatexchanger 115, the half sections 125a and 125b can be opened (FIG. 3)and the exposed surfaces of the tubular coil 130 and the inner and outersleeves 120 and 125 respectively, can be thoroughly, quickly and easilycleaned. By maintaining the heat exchanger 115 relatively clean, itfollows that the efficiency of the heat exchanger is maintained.

The present invention may, of course, be carried out in other specificways than those herein set forth without parting from the spirit andessential character of the invention. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

What is claimed is:
 1. A pre-digestion pathogen reduction system fortreating sludge prior to the sludge being directed into a digester in awastewater treatment system, comprising:a) an inlet for directing theincoming sludge into the system; b) a series of reactors for receivingthe incoming sludge as a series of batches, holding the batches ofincoming sludge for a predetermined time to produce a pathogen-reducedsludge, and discharging the batches of treated sludge in a manner so asto form a continuous outflow of treated sludge to the digester, eachreactor being operative to assume the feeding, holding, and dischargemodes in sequence; c) a sludge-to-sludge heat exchanger disposed on aninlet side of the reactors for receiving both the incoming sludge andthe treated pathogen reduced sludge and transferring the heat associatedwith the treated sludge to the incoming sludge; d) an outlet fordirecting the treated sludge from the reactors to the sludge-to-sludgeheat exchanger prior to the sludge being directed to the digester suchthat as the treated sludge passes through the sludge-to-sludge heatexchanger, the heat associated therewith is transferred to the incomingsludge so as to heat the incoming sludge while at the same time coolingthe treated sludge; and e) at least one additional heat exchanger forcooling the treated sludge leaving the reactors, the additional heatexchanger being a wastewater effluent heat exchanger that is coupled toa source of wastewater effluent wherein the wastewater effluenteffectively cools the treated sludge.
 2. The system of claim 1 whereineach reactor at any one time is operative to assume a feeding, holding,or discharge mode with respect to the sludge being treated.
 3. Thesystem of claim 1 wherein a heating heat exchanger is communicativelyconnected to an external source of heat and to the incoming sludge suchthat the external source of heat is utilized to heat the sludge prior tothe sludge entering the reactors.
 4. The system of claim 1 wherein thereactors are disposed in parallel relationship and operate to treat thesludge in batch form but yet continuously direct sludge from thereactors to the digester.
 5. The system of claim 1 wherein there isprovided a control valve on both the inlet and outlet sides of eachreactor for controlling the flow of sludge into and from the reactor. 6.The system of claim 5 including a programmable controller forcontrolling the control valves and controlling the flow of sludge to andfrom the respective reactors.
 7. The system of claim 6 including atleast three reactors disposed in parallel relationship, and wherein theprogrammable controller in conjunction with the control valves directsludge into one reactor while directing sludge from a second reactorwhile sludge is being held in the third reactor.
 8. The system of claim7 wherein the programmable controller in conjunction with the controlvalves is operative to sequentially change the mode of operation of eachreactor such that during the course of treating the sludge for pathogenseach reactor cycles between three separate modes of operation, a fillingmode, a holding mode, and a discharge mode.
 9. The system of claim 8wherein the controller is a programmable controller and wherein theprogrammable controller in conjunction with the control valves controlsthe mode of each reactor such that in general each reactor cyclesthrough a series of different modes of operation during the pathogenreduction process.
 10. A system for reducing the pathogens in sludgecomprising: a sludge inlet line; a series of batch reactors connected tothe sludge inlet line for receiving incoming sludge therefrom, theseries of batch reactors disposed in parallel relationship and whereinduring a selected treatment interval one reactor assumes a holding andtreating mode and produces treated sludge while the other reactorassumes a discharge mode; a series of heat exchangers for heatingincoming sludge and cooling the treated sludge including at least onesludge-to-sludge heat exchanger operatively connected between the sludgeinlet line and the series of reactors for heating incoming sludgepassing in the inlet line and for cooling treated sludge leaving thereactors and wherein the sludge-to-sludge heat exchanger is connected tothe inlet line such that incoming sludge passing through the inlet lineis directed through the sludge-to-sludge heat exchanger before reachingthe reactors; and wherein the system includes an outlet line fordirecting treated sludge from the reactors back to and through thesludge-to-sludge heat exchanger such that the treated sludge passingfrom the outlet line back through the sludge-to-sludge heat exchangeracts to heat the incoming sludge while at the same time the treatedsludge is cooled; and a series of control valves disposed on inlet andoutlet sides of the reactors and a programmable controller operativelyconnected to the control valves for controlling the same such that theindividual reactors can be controlled to operate in a batch mode whilethe series of reactors as a group can be controlled to provide acontinuous flow of sludge through the reactors.
 11. The system of claim10 wherein the pathogen reduction system is coupled to a digester thatreduces the volatile solids within the sludge while the pathogenreduction system reduces the pathogen concentration within the sludge.12. The system of claim 10 wherein the series of reactors include threereactors disposed in parallel relationship with each reactorsequentially assuming a filling, holding and discharging mode andwherein the operating modes of the three reactors are staggered suchthat generally the individual reactors assume different operating modes.13. The system of claim 10 including a series of additional heatexchangers that form a part of the system and which function to furtherheat and cool the treated sludge and the incoming sludge.
 14. The systemof claim 13 wherein the series of heat exchangers includes a heatexchanger operatively connected to a wastewater effluent supply and tothe treated sludge exiting the reactors for effectively cooling thetreated sludge, and wherein the series of heat exchangers includesanother heat exchanger connected to an external source of heat andoperatively associated with the incoming sludge being directed to thereactors for providing additional heat to the incoming sludge.
 15. Amethod of treating sludge to reduce the pathogen concentration of thesludge comprising:a) heating incoming sludge by directing the incomingsludge into and through a sludge-to-sludge heat exchanger; b) directingthe heated incoming sludge to a series of reactors; c) during one phase,holding the heated sludge within one reactor for a selected time periodwhile discharging heated sludge from another reactor; d) during anotherphase, discharging the heated sludge from the one reactor while holdingthe sludge in the other reactor, thereby processing the heated sludge inbatch form while providing a continuous outflow of treated sludge fromthe reactors; e) directing the treated sludge from the reactors back tothe sludge-to-sludge heat exchanger and routing the treated sludge backthrough the sludge-to-sludge heat exchanger such that heat associatedwith the treated sludge heats the incoming sludge and in the process theheated sludge is cooled; f) further cooling the treated sludge bydirecting the treated sludge to a separate cooling heat exchanger; andg) directing treated wastewater effluent to the cooling heat exchangerand utilizing the treated effluent to cool the treated sludge after thetreated sludge has exited the reactors.
 16. The method of claim 15including directing the incoming sludge through a second heat exchangerthat is operatively coupled to an external source of heat.
 17. Themethod of claim 15 including directing the continuous outflow of treatedsludge from the reactors to a digester for further treatment.
 18. Themethod of claim 15 including heating the incoming sludge to atemperature of approximately 60 to 70° C.
 19. The method of claim 15wherein the reactors include three reactors disposed in parallelrelationship with each reactor assuming a sludge filling, holding anddischarging mode during certain times of the pathogen reduction process,and wherein the operation mode the three reactors are staggered suchthat generally the three reactors are operating in different modes atany one time such that the sludge is processed in batch form butcontinuously fed to and from the reactors.
 20. The method of claim 19including holding the sludge within the reactors for approximately onehour.
 21. The method of claim 15 including locating the cooling heatexchanger downstream from the sludge-to-sludge heat exchanger such thatthe treated sludge is cooled by the effluent after it has been firstcooled by the sludge-to-sludge heat exchanger.
 22. The method of claim21 including direct the incoming sludge through a second heating heatexchanger prior to the incoming sludge reaching the reactors, andheating a system of water and directing the heated water to the secondheating heat exchanger so as to provide additional heat to the incomingsludge prior to reaching the reactors.
 23. The method of claim 15wherein there is provided a control valve on both the inlet and outletsides of each reactor and wherein the control valves and the flow ofsludge through the reactors are controlled by a programmable controller.